Monoclonal antibody therapy directed at the alpha chain (TAC/CD25) of the high affinity IL-2 receptor (IL-2R) is a growing therapy in transplantation and autoimmune disease. However, the mechanisms by which this therapy may limit immune responses have not been fully elucidated. Using a humanized anti-TAC antibody in vitro, we investigated the role of IL-2R blockade in the regulation of Thl immune responses. Our preliminary data indicate HAT inhibits production of the Thl effector cytokine, IFN-y, and the central regulatory Thl cytokine, IL-12, from activated peripheral blood mononuclear cell (PBMC) cultures. We hypothesize CD40 ligand (CD40L)/CD40 interactions play a major role in HAT-mediated inhibition of IL-12- dependent Thl responses and show CD40L expression on activated T cells is biphasic, with HAT inhibiting late phase CD40L. In SA#1, we will extend this studies to determine whether differential regulation of CD40L on naive and memory T cells by CD28 costimulation and/or IL-2R signaling accounts for biphasic expression. We will also determine the role of IL-2R signaling in regulating IL-12R expression and IL-12 responsiveness in T cells, since IL-12 cannot restore IFN-_/in the presence of HAT. In SA#2, we will focus on the molecular mechanisms by which IL-2R blockade and/or CD28 costimulation regulates CD40L gene expression. Using a 1.3Kb CD40L promoter reporter construct and transiently transfecting a Thl cell clone, we will test putative Stat5 binding elements to determine whether IL-2R blockade regulates CD40L expression at these sites. In SA#3, we hypothesize that HAT directly inhibits IFN-_, production from T cells in an IL-12-independent, cell cycle-dependent mechanism. To test this, we will assess the effects of HAT on cell cycle progresson, using carboxyfluorescin diacetate succinimidyl ester (CFSE)-Iabeling and co-staining for IFN-y in activated T cells subjected to cell cycle arrest at various stages. The PI, Dr. John McDyer is a junior faculty member and committed to a career in academic medicine and understanding the mechanisms through which anti-TAC .regulates human immune responses; this knowledge will improve our understanding of the role of IL-2R blockade in human T cell responses and differentiation and may expand the role for HAT antibody therapy in the treatment of immune-mediated diseases. This K08 will provide the foundation for a research career dedicated to translational studies in immunology.